1. Field of the Invention
The present invention relates to ultrasonic sensors, and more particularly, to a ultrasonic sensor used, for example, for a back-up sensor on an automobile.
2. Description of the Related Art
FIG. 12 is a schematic diagram showing an example of a known ultrasonic sensor. An ultrasonic sensor 1 shown in FIG. 12 includes a case 2 having a substantially cylindrical shape with a bottom, the case 2 being formed of aluminum or other suitable material. One surface of a piezoelectric element 3 is bonded to an inner bottom surface of the case 2. The inside of the case 2 is substantially entirely filled with foamable resin 4, such as foamable silicon so as to cover the piezoelectric element 3. A substrate 6 having two terminals 5a and 5b is attached in an opening portion of the case 2 so as to cover the foamable resin 4. On opposed surfaces of the substrate 6, electrodes 7a and 7b are connected to the terminals 5a and 5b. The terminal 5a is connected to an opposite surface of the piezoelectric element 3 by the electrode 7a formed on the inner side of the substrate 6 and a wire 8. The terminal 5b is connected to the other surface of the piezoelectric element 3 by the electrode 7b formed on the outer side of the substrate 6 and solder 9 through the case 2.
Where a distance to an object to be detected is measured using the ultrasonic sensor 1 shown in FIG. 12, a driving voltage is applied to the terminals 5a and 5b to excite the piezoelectric element 3. As the piezoelectric element 3 is vibrated, the bottom surface of the case 2 is also vibrated, which causes an ultrasonic wave to be emitted in a direction perpendicular to the bottom surface as indicated by an arrow shown in FIG. 12. When the ultrasonic wave emitted from the ultrasonic sensor 1 is reflected by an object to be detected and reaches the ultrasonic sensor 1, the piezoelectric element 3 is vibrated and the vibration is converted into an electrical signal. The electrical signal is output from the terminals 5a and 5b. A period of time from the application of the driving voltage to the output of the electrical signal is measured, and thus, the distance from the ultrasonic sensor 1 to the object to be detected can be measured.
In the ultrasonic sensor 1 shown in FIG. 12, since the inside of the case 2 is filled with the foamable resin 4, the vibration of the overall case 2 is suppressed. Further, the ultrasonic wave generated in the inside of the case 2 is scattered and absorbed by a large number of foam pores in the foamable resin 4. This can efficiently suppress the vibration of the case 2 and the ultrasonic wave propagation within the case 2, thereby improving reverberation characteristics (see, for example, Japanese Unexamined Patent Application Publication No. 11-266498).
FIG. 13 is a schematic diagram showing another known ultrasonic sensor. An ultrasonic sensor 1 shown in FIG. 13 is different from the ultrasonic sensor 1 shown in FIG. 12, in that a substrate 6 having terminals 5a and 5b is not directly attached to a case 2, but instead, is covered with an elastic member 4a composed of silicon rubber or other suitable material which is filled in the inside of the case 2. A sealant 4b composed of a high-gas-barrier silicon material is provided on an exposed surface of the elastic member 4a. The terminal 5a is connected to a piezoelectric element 3 through a lead wire 8a, and the terminal 5b is connected to the piezoelectric element 3 through a lead wire 8b and the case 2.
In the ultrasonic sensor 1 shown in FIG. 13, the sealant 4b composed of a high-gas-barrier silicon material provided on the exposed surface of the elastic member 4a can effectively prevent corrosive gas from entering the case 2. Furthermore, the silicon material used as the sealant 4b can ensure a sufficient level of reverberation characteristics even in a cold temperature environment with rapid temperature changes (see, for example, Japanese Unexamined Patent Application Publication No. 2001-197592).
In the ultrasonic sensor 1 shown in FIG. 12, the substrate 6 having the terminals 5a and 5b is fixed in direct contact with a side surface of the case 2. Thus, the vibration of the piezoelectric element 3 is transmitted to the case 2 and the substrate 6 and is damped from the terminals 5a and 5b. 
On the other hand, in the ultrasonic sensor 1 shown in FIG. 13, since the substrate 6 having the terminals 5a and 5b is covered with the elastic member 4a composed of silicon rubber or other suitable material which is filled in the inside of the case 2, vibration damping through the case 2 is less likely to occur.
However, in the ultrasonic sensor 1 shown in FIG. 13, the positions of the substrate 6 and the terminals 5a and 5b are often displaced by a filling process of silicon rubber or other suitable material used for the elastic member 4a, and it is difficult to position the terminals 5a and 5b at desired positions in the ultrasonic sensor. This may result in failure during mounting.
In the ultrasonic sensors 1 shown in FIGS. 12 and 13, the wire 8 and lead wires 8a and 8b inside the case 2 are bonded to the terminals 5a and 5b by soldering, welding, or other suitable method. However, due to the configuration of ultrasonic sensors 1 shown in FIGS. 12 and 13, the substrate 6, the case 2, and other components are assembled after the bonding. Thus, the length of the wire 8 and the lead wires 8a and 8b must be increased. Further, an excessive stress may be applied to the bonding portions of the wire 8 and the lead wires 8a and 8b when the case 2 and other components are assembled. Therefore, the wiring of the terminals 5a and 5b is very difficult.